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GPS Interference By Solar Radio Bursts

Solar interference to GPS downlink signals appears to be a
possibility during the lock acquisition phase, but the probability is
very low, and is expected to occur only a few times in each solar cycle (around
11 years in duration) and only for a few minutes on each occasion.

To examine link margins, consider the following figures for a typical
small handheld GPS navigator. With an antenna gain (G) of one (0 dBi),
the typical GPS power at the receiver input is around -160 dBW. A typical
system noise temperature (Ts) of 350 degrees K (this includes ambient noise plus
receiver noise). gives an equivalent input noise power over a 1 MHz
bandwidth of -143 dBm (GPS C/A signals are spread over 2 MHz but most of
the power is concentrated within a 1 MHz bandwidth). The signal to noise
ratio (SNR) at the receiver input is thus about -17 dB. However, during
initial acquisition of the GPS, the correlation loop reduces the bandwidth
to about 1 KHz providing a 30 dB increase in SNR. During the acquisition
phase the SNR is therefore about +13 dB. Once tracking the signal,
the bandwidth is further reduced to about 50Hz which provides an additional
increase of 13 dB, giving a total tracking SNR of +26 dB.

The background solar flux at L-band varies from 50 to 150 SFU
(solar flux unit)
over the solar cycle. This is much too low
to cause interference to GPS signals. However, during certain explosive
events on the Sun, bursts of radio energy are produced which vastly
exceed this value. The maximum burst component (Sb) recorded at L-band is
around 100,000 SFU. The decrease in SNR produced by a solar noise
burst is given by equation:

Change in SNR (dB) = 10 log 10 [1.0 + ( Sb G c2 ) / ( 4 pi f2 k Ts) ]

where Sb is the maximum burst in SFU, G is the antenna gain, c is the speed of light, f is the frequency (1575
MHz), k is the Boltzmann constant and Ts is the system noise temperature.

The table below lists the SNR decrease (in dB) for GPS receivers with different
antenna gain and system noise temperature combinations. For the receiver
example given above an 8 to 9 dB reduction can be expected for this
magnitude solar burst. This is likely to make signal acquisition very
difficult if not impossible, but is unlikely to cause signal loss when
loop lock has been established. However, the link margin will be reduced,
and this, in combination with other effects such as hi-g maneuvers or
ionospheric scintillations may affect system performance. Such large
solar bursts are rare. In solar cycle 20 (1965-1976) one such burst of
165,000 SFU occurred on 29 April 1973. Total burst duration was 40 minutes
although the time above 100,000 SFU was much shorter. In December 2006
radio noise bursts exceeding 100,000 SFU and maybe as high as 1,000,000 SFU
were recorded at the SWS/USAF Learmonth Solar Observatory (see below).

Solar Burst Interference to GPS Signals
Decrease in SNR for a solar burst of 100,000 SFU, based on antenna gain and system noise temperature.

Antenna Gain (dB)

50K

100K

150K

200K

250K

300K

350K

400K

450K

500K

0

16.3

13.4

11.7

10.6

9.7

9.0

8.4

7.9

7.5

7.1

1

17.3

14.4

12.7

11.5

10.6

9.9

9.3

8.8

8.4

8.0

2

18.3

15.3

13.6

12.4

11.5

10.8

10.2

9.7

9.2

8.8

3

19.3

16.3

14.6

13.4

12.5

11.7

11.1

10.6

10.1

9.7

4

20.3

17.3

15.6

14.4

13.4

12.7

12.0

11.5

11.0

10.6

5

21.2

18.3

16.5

15.3

14.4

13.6

13.0

12.4

12.0

11.5

6

22.2

19.3

17.5

16.3

15.4

14.6

13.9

13.4

12.9

12.5

7

23.2

20.2

18.5

17.3

16.3

15.6

14.9

14.3

13.9

13.4

8

24.2

21.2

19.5

18.3

17.3

16.5

15.9

15.3

14.8

14.4

9

25.2

22.2

20.5

19.2

18.3

17.5

16.9

16.3

15.8

15.3

10

26.2

23.2

21.5

20.2

19.3

18.5

17.8

17.3

16.8

16.3

11

27.2

24.2

22.5

21.2

20.3

19.5

18.8

18.3

17.7

17.3

12

28.2

25.2

23.5

22.2

21.3

20.5

19.8

19.2

18.7

18.3

13

29.2

26.2

24.5

23.2

22.3

21.5

20.8

20.2

19.7

19.3

14

30.2

27.2

25.5

24.2

23.2

22.5

21.8

21.2

20.7

20.3

15

31.2

28.2

26.5

25.2

24.2

23.5

22.8

22.2

21.7

21.2

16

32.2

29.2

27.5

26.2

25.2

24.5

23.8

23.2

22.7

22.2

17

33.2

30.2

28.5

27.2

26.2

25.4

24.8

24.2

23.7

23.2

18

34.2

31.2

29.4

28.2

27.2

26.4

25.8

25.2

24.7

24.2

19

35.2

32.2

30.4

29.2

28.2

27.4

26.8

26.2

25.7

25.2

20

36.2

33.2

31.4

30.2

29.2

28.4

27.8

27.2

26.7

26.2

21

37.2

34.2

32.4

31.2

30.2

29.4

28.8

28.2

27.7

27.2

22

38.2

35.2

33.4

32.2

31.2

30.4

29.8

29.2

28.7

28.2

23

39.2

36.2

34.4

33.2

32.2

31.4

30.8

30.2

29.7

29.2

24

40.2

37.2

35.4

34.2

33.2

32.4

31.8

31.2

30.7

30.2

25

41.2

38.2

36.4

35.2

34.2

33.4

32.8

32.2

31.7

31.2

26

42.2

39.2

37.4

36.2

35.2

34.4

33.8

33.2

32.7

32.2

27

43.2

40.2

38.4

37.2

36.2

35.4

34.8

34.2

33.7

33.2

28

44.2

41.2

39.4

38.2

37.2

36.4

35.8

35.2

34.7

34.2

29

45.2

42.2

40.4

39.2

38.2

37.4

36.8

36.2

35.7

35.2

30

46.2

43.2

41.4

40.2

39.2

38.4

37.8

37.2

36.7

36.2

Reference: John A Kennewell, Solar Radio Interference to Satellite
Downlinks, Proceedings ICAP 89, Sixth International Conference on
Antennas and Propagation, vol 2, pp334-9 (IEE London 1989).

Epilogue

In December 2006 a number of radio noise bursts occurred associated with X-class solar x-ray flares.
The one recorded at the SWS/USAF Learmonth Solar Observatory on 13 December saturated the receiver
at levels exceeding 100,000 SFU. A detailed study of GPS performance was conducted by SWS.
Loss of lock with GPS satellites was experienced for around 2 hours following the
event and for some receivers a complete loss of navigation solution experienced for 6 - 10 minutes.